This section is intended to provide a background to the various embodiments of the technology described in this disclosure. The description in this section may include concepts that could be pursued, but are not necessarily ones that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, what is described in this section is not prior art to the description and/or claims of this disclosure and is not admitted to be prior art by the mere inclusion in this section.
The ultimate goal of mobile broadband should be the ubiquitous and sustainable provision of non-limiting data rates to everyone and everything at every time. Ultra-Dense Networks (UDN) is an important next step following the successful introduction of Long-Term Evolution (LTE) for wide-area and local-area access (referring to “3GPP TS 25.302: “Services provided by the Physical Layer”). One example of UDN is a MilliMeter-Wave (MMW) Radio Access Technology (RAT) network.
FIG. 1 schematically shows one example MMW RAT network. As shown in FIG. 1, there is a network node or a control node called as Central Control Unit (CCU), which is responsible for parameter configurations and coordination among Access Points (APs) or Access Nodes (ANs), e.g., AN1, AN2, AN3, and AN4, or any other radio nodes that enable of covering a certain geographical area. Each AN can serve one or more communication devices, such as User Equipments (UE), operating in the wireless communication networks or systems, also known as e.g. wireless terminals, mobile terminals and/or mobile stations, referred to as terminal devices hereafter. For example, AN1 serves UE1, and AN2 serves UE2, etc.
Even though only a local-area access technology, UDN can be deployed in areas with high traffic consumption and thus provide an important step towards the above goal. Through overprovision and the related low average loads in the access network, UDN can create ubiquitous access opportunities, which provide users with the desired data rates even under realistic assumption on user density and traffic. Overprovision is achieved by an extremely dense grid of ANs, e.g., as illustrated in FIG. 1. Inter-AN distances in the order of tens of meters and even below are envisioned in indoor deployments where one or even multiple ANs are conceivable in each room. In addition to the increased network capacity, densification—via reduced transmit powers—also offers access to vast spectrum holdings in the MMW bands and thus increased data rates.
For example, several GHz of spectrum is available in the unlimited 60 GHz band and potentially more in other millimeter-wave bands enabling multi-Gb/s transmissions even with technologies providing moderate spectral efficiency. While maybe perceived as old-fashioned, schemes with moderate spectral efficiencies offer robustness and energy efficient data transmission. Furthermore, there are also implementation issues at higher MMW frequencies that make it very challenging to provide very high spectral efficiency (in b/s/Hz). In this sense, one can trade spectral efficiency for bandwidth.
FIG. 2 shows the traditional handover procedure in LTE. As shown in FIG. 2, the handover procedure consists of the following steps:                Initially, UE needs to perform measures on both its serving cell and one or more neighboring cells. For example, UE may measure signal strengths from these cells. Once measurement results meet a handover criterion for more than a Time-to-Trigger (TTT) duration, Event A3 is triggered (referring to “LTE—The UMTS Long Term Evolution From Theory to Practice”).        If Event A3 is triggered, measurement reports can be sent from UE to its source eNB corresponding to the serving cell.        When source eNB receives the measurement reports, source eNB can select the right target eNB and then exchange handover related information with the selected target eNB.        Then, source eNB can send Handover (HO) Command to UE and ask UE to do handover.        Finally, UE initiates random access to the target cell and finally sends Handover Complete to target cell.        
For UE mobility within an MMW-RAT network, the traditional handover procedure as shown in FIG. 2 is not suitable as source AN (also referring to as serving AN) needs to know the neighborhood relationship with target AN and needs to forward both context information and packets toward target AN. This means quite a lot burden and overhead on each AN. Instead, it has been proposed that the mobility management is controlled by a Network Controller (NC). As NC is the CCU in the mmW network, it knows the network topology very well. NC could know which other neighbor AN may need to serve the UE when it connects with one serving AN.
FIG. 3 shows the mobility procedure in MMW-RAT network.
As shown in FIG. 3, when a UE connects with a source AN, it will report measurement results to the NC. Then the NC can determine if another AN nearby the source AN needs be prepared to serve the UE or not, and if the UE need to switch to the target AN or not. If the UE needs to switch to the target AN, different from the traditional handover, the source AN is not required to transfer the UE context to the target AN, and forward packets toward the target AN. Instead, the target AN obtains the necessary context information from either the UE or from the NC. The information to be acquired from the UE can be UE network capability, the packet status information, UE historical information, etc. The information to be acquired from the NC can be the bearer or Quality of Service (QoS) related information. Since the security is between UE and Local-GateWay (L-GW)/NC, neither the source AN nor the target AN needs to care about this issue, like they would in traditional handover.
A UE-specific serving cluster (SvC) is a group of ANs that are located in the vicinity of a UE and are ready to serve the UE. To control fast beam switch, a cluster head (CH, also called as a cluster manager) is needed for coordination in the cluster. Cluster head might not be located in different nodes, so the cluster head concept can be used in the structures of different backhauls. For wired backhaul, if it is centralized coordination, cluster head is located in the CCU. If it is distributed coordination, cluster head is located with one AN (in this case, it is the definition of P-SAN). For wireless self backhaul, cluster head's location depends on the topology of the cluster. It may or may not be a P-SAN depending on topology and UE position. Besides the coordination of beam switch, cluster head handles the majority of data to be sent to and to be received from the UE. For wireless self-backhaul, in order that the cluster head can coordinate the inter-AN beam switch fast, it is assumed that there is only one hop between cluster head and ANs in the cluster.
FIG. 4 illustrates an exemplary deployment of the MMW-RAT system. As shown in FIG. 4, (AN1, AN2 and AN3) can be in one cluster (Cluster 1), (AN3, AN4 and AN5) can be in one cluster (Cluster 2). Apparently, AN1 serves a cluster head of Cluster 1, and AN3 serves a cluster head of Cluster 2. AN2 and AN4 cannot be in one cluster because there are more than one hops between AN2 and AN4.
Typically, measurement reports obtained by UE are Layer 1 (L1)/Layer 2 (L2) measurements. However, as illustrated in FIG. 2 or FIG. 3, no matter during the traditional handover procedure in LTE or during the mobility procedure in MMW-RAT network, it is Layer 3 (L3) measurements to be sent by UE to source eNB/NC. Therefore, UE has to convert L1/L2 measurements to L3 measurements in advance. In other words, UE has to perform post-processing of L1/L2 measurements to get L3 measurements, and then send L3 measurements to source eNB/NC for corresponding mobility procedure.